Xerographic developer



April 2, 1968 R. H. EICHORN I 3,375,807

XEROGRAPHIC DEVELOPER 5 Sheets-Sheet 1 Filed Oct. 22, 1965 INVENTOR.

ROGER H. EICHORN April 2, 1968 R. H. EICHORN 3,375,807

XEROGRAPHIC DEVELOPER Filed Oct. 22, 1965 s Sheets-Sheet 'NVENTOR. ROGER H. E|CHORN April 1968 R. H EICHOF QN 3,375,807

I XEROGRAPHIC DEVELOPER Filed Oct. 22, 1965 3 Sheets-Sheet INVENTOR. ROGER H. EICHORN A TTORNE rs United States Patent ()fiice 3375307 XEROGRAPHIC DEVELGPER Roger H. Eichorn, Webster, N.Y., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Get. 22, 1965, er. No. 501,206 5 Claims. (Cl. 118-637) ABSTRACT OF THE DHSCLGSURE A device for developing a latent electrostatic image on a xerographic surface including an impeller mounted adjacent the xerographic drum for agitating and impelling carrier and powder against the xerographic plate.

This invention relates to the field of xerography and particularly to apparatus for developing an electrostatic latent image on a xerographic plate. More specifically, the invention relates to a xerographic development system which produces powder images on a moving xerographic plate by bringing a two-component developer material into contact with the moving xer-ographic plate at controlled relative speeds.

In the process of xerography, as disclosed in Carlson Patent 2,297,691 issued Oct. 6, 1942, a xerographic plate comprising a layer of photoconduct-ive insulating material on a conductive backing is given a uniform electrostatic charge over its surface and is then exposed to the subject matter to be reproduced, usually by conventional projection techniques. This exposure discharges the .plate areas in accordance with the light intensity that reaches them and thereby creates an electrostatic latent image on or in the photoconductive layer. Development of the latent image is effected with an electrostatically charged, finely divided material such as an electroscopic powder that is brought into surface contact with the photocondu-ctive layer and is held thereon electrostatically in a pattern corresponding to the electrostatic latent image. Thereafter, the developed xerographic powder image is usually transferred to a support surface to which it may be fixed by any suitable means.

In the Carlson patent it is noted that a variety of types of finely divided electroscopic powders may be employed for developing electrostatic latent images. However, as the art of xerograe hy has progressed, it has been found preferable to develop line copy images with a two-component type of developing material such as that disclosed in Walkup Patent No. 2,618,551 issued Nov. 1 8, 1952, which may include a powder or toner of any of a variety of pigmented resin that has been specifically developed for that purpose, and a granular carrier material, such as glass, encased in a suitable covering such as, for example, a polymer capable of establishing a desired tri'boelectric relationship with the toner. A number of such developing materials are manufactured and marketed commercially, being specifically compounded for producing dense images of high resolution and to have characteristics to permit convenient storage and handling.

Generally, in a xerographic reproduction a latent electrostatic image is developed into a powder image by cascading the developing material over the surface of the xerographic plate. Carrier beads and toner powder are mixed together and agitated so that the triboelectric charge is formed on the carrier beads and toner particles to attract and hold toner particles to the carrier beads. The combination of carrier beads and toner powder is referred to herein as developer. The developer material is then elevated to a position where it may be allowed to freely fall or cascade over the surface of a xerographic plate. This is generally referred to in the art as cascade development.

3,375,807 Patented Apr. 2, lfifi In automatic xerographic machines, the xerographic plate is usually in the form of a cylindrical drum, and the developer material is allowed to cascade over the surface of the drum as the drum rotates. While the developer material is in moving contact with the surface of the xerographic plate, the electrostatic charge on the plate is greater than the tri-boelectric charge holding the toner on the carrier beads so that the toner particles are attracted to the plate surface away from the carrier beads. The powder remains on the are-as of the plate that have the electrostatic charge in image configuration, and the carrier falls from the plate surface and is remixed with toner and recirculated.

Cascade development has been one of the most etfective forms of developing systems used in the xerographic process. However, even though cascade development produces excellent powder images and has been commercially successful, there are limitations which restrict the speed at which the xerographic plates may be moving. Further, in cascade development large areas of the plate surface are required to effect proper development. For example, in conventional cascade development the velocity of the developer material while it is cascading over the plate surface is limited by the gravita-tional efiect on a freely falling body. In order to increase the velocity of the carriers, it is necessary to extend the point at which the carriers are allowed to fall before encountering the xerographic plate. It has been found that to produce optimum development, the speed of the carriers relative to the speed of the plate should be approximately 32 inches per second. In order to increase the number of feet "of xerographic plate developed in a given time and still maintain the optimum relative speed between the developer and the plate, it is necessary to increase the speed of the developer. As pointed out above, under a gravity system the speed can be increased only by increasing the size of the developing unit to give the developer a longer fall before encountering 'the xerographic plate. Since the movement of the developer is dependent on gravity, the position of the development unit is restricted to areas of the drum where gravity will carry the material over the drum surface. P or example, conventional cascade development cannot be effected on the bottom of the drum.

Conventional cascade is used primarily for the development of line copy. Development of solid areas or of half tone and tone images can be produced most elfectively with powder cloud development. Powder cloud is the process of bringing an electrostatic image .into contact with powder or toner material suspended in air. The electrostatic charge on the photoconductive surface attracts the powder out of the air and deposits it on the photoconductive surface in image configuration. Since there is no triboelectric charge to overcome, sma ler electrostatic charges will attract the powder. Also, however, small residual charges in non-image areas will attract powder producing darker non-image areas commonly called background. In two-component development, the carrier particles which roll across the non-image areas tend to pick up any powder which has been deposited in the nonimage or background areas. This is refer-red to as scavenging. These is no scavenging effect with powder cloud, so consequently there is greater background.

It is, therefore, an object of this invention to develop xerographic images.

It is an object of this invention to develop xerographic images using two-component developer.

It is an object of this invention to improve xerographic development systems to allow increased surface speeds of a moving xerograp-hic plate.

Further objects of this invention are to reduce the size of two-component developer systems, to attain a greater .3 flexibility in the speed of developer material and to provide a two-component developer system that is not dependent on gravity feed.

It is also an object of this invention to develop electro static images with two-component developer and powder cloud development simultaneously.

These and other objects of this invention are obtained by means of developer impellers mounted in a housing containing a supply of developer material and positioned to impel developer material against the surface of a xero graphic plate.

For a better understanding of the invention, as well as other objects and further features thereof, reference is had to the following detailed description of the invention to be read in connection with the accompanying drawirlgs, wherein:

FIG. 1 is a schematic right side sectional view of the machine showing the invention;

FIG. 2 is a perspective view of the xcrographic drum, developer unit and toner dispenser of the machine shown in FIG. 1 with portions broken away to show internal construction;

FIG. 3 is a front view of the toner dispenser shown in FIG. 2;

FIG. 4 is an embodiment of the developer unit having two paddle wheels;

FIG. 5 is an embodiment of the development unit having one paddle Wheel; and

FIG. 6 is another embodiment of the development unit having one paddle wheel.

The xerographic machine shown in FIG. 1 has a front opening 16 to receive an original document to be reproduced, and an opening 18 into which copy paper or support material is fed to receive a powder image. After an image has been transferred to the copy paper, the paper emerges from a slot Ztl in the top cover M.

A xerographic drum 2?; having a layer of photoconductive material 23 about its outer surface is mounted for rotation on a shaft 24. As the drum 22 rotates, it moves past a charging station 26 wherein a uniform electrostatic charge is placed on the photoconductor 23 by means of a corotron 28. The drum then rotates past an exposure station 30 and a developer station 32. At the developer station 32, a powder image is formed corresponding to a latent electrostatic image created on the drum surface at the exposure station. The light image at exposure station 30 discharges the drum in areas of light and leaves the drum an insulator in areas of darkness thereby creating an electrostatic latent image on the drum surface. As the drum rotates through the developing station 32, a paddle wheel 34 impels developer material 36 against the surface of the drum 22. The developer material is composed of a granulated carrier bead and resinous powder or toner material used to develop the electrostatic images. The toner material is triboelectrically held on the carrier beads and is electrostaticaliy attracted away from the carrier beads to the areas containing the latent electrostatic image, thereby developing a powder image.

The drum continues to rotate past a transfer station 38 having a transfer corotron 4t and a drum cleaning station 42 having a precleaned corotron 44 and a web cleaner 46. The powder image on the drum surface is transferred to a sheet of paper or support material at the transfer station 38. The transfer corotron is positioned to provide an electrostatic charge on the back of a belt 48. The belt 48 is of dielectric material and moves in a continuous loop so that sheets of paper will be carried into contact with the surface of the drum 22. A sheet of paper inserted in the opening 18 is forced forward past an electrostatic charging device 49 until it trips a switch LS1. The switch LS-l turns on the corotron 49 which tacks the paper to the belt. The transfer corotron 40 then applies a charge to the back of the belt. The electrostatic charge placed on the back of the belt by the electrostatic charging device or corotron 4t) attracts the powder image from the drum surface to the surface of the paper. The paper is then carried forward past a laser 50 wherein the powder is heated and caused to coalesce and bond to the surface of the paper. The paper is gripped by a pair of ejection rollers 52 and driven forward against a deflec ion guide 54 which curves the paper upwards and out slot 2.0 to the top of the machine.

The toner powder used herein has a negative charge relative to the various charges used to create the latent electrostatic images and to transfer the powder from the drum to the support material surface. The latent electrostatic image is created from the positive charge that is placed on the surface of the xerographic drum by the corotron 23. The negative charged toner is attracted to r the positive latent electrostatic image. At the transfer station, the transfer corotron 41) places another positive charge on the back of the transfer material and the belt 48 so that the negative toner material is then attracted to the surface of support material.

The drum cleaning station 42 lus a cleaning roller 56 which forces a web of fibrous material 46 into surface contact with the rotating xerographic drum. The residual powder which has not been transferred to the sheet of paper or support material remains on the drum surface and must be removed prior to the creation of a new image on the surface of the drum. The powder material is held on the drum surface by electrostatic charges which originally were used to create the image and residual electrostatic charges in non-image areas. The effect of these electrostatic charges is neutralized by the use of the prcclean corotron 44 which deposits a uniform electrostatic charge of opposite polarity to that holding the toner on the surface of the drum. The fibrous web material 46 is taken from a supply roll 58 around the cleaning roll 56 into contact with the surface 23 of the drum 22 and onto a take-up roll 60. The surface of the xcrographic drum is then exposed to a spot of light 62. which discharges the drum. surface, removing any residual electrostatic charges which may exist on the drum surface.

The original copy to be reproduced is positioned on a counter 64 and inserted into the document opening or slot 16. The document slides forward, deflecting switch actuator 66 of a limit switch until it encounters the document positioning stop 68. With the document abutting against the stop 68, it overlies a feed belt system 71) and is beneath a feed roller '72. When the machine is in operation, as described below, the feed roller 72 comes down on the document, forcing it against the feed belt 76 and driving it forward. The stop 68 which is mounted on a shaft 74 is rotated in a counterclockwise direction as seen in FIG. 1, deflecting the front end of the document down along the path of the feed belt 70. The belts are constructed of a dielectric material and the document is held in contact with the belts 70 by a row of leaf spring-type guides 76 which extend across the conveyor system. The document is electrostatically tacked to the belt 70 by electrostatic charge applied to the document surface by a corotron 78.

The feed roll belts '70 extend around belt rollers 80 and are held in tension by a pair of guide rollers 82, one of which is adjustable to vary the tension on the belts 70. The documents are carried by the belts 70 past scanning station 84 where they are illuminated by a pair of lamps LMP1. The documents are carried down between the belts 7tl and a lower roller 86 at the same speed as the surface of the xerographic drum and then ejected through a slot 88 at the bottom front of the machine. The belts 7t and the documents held thereon are flat against a backup platen 9th at the scanning station. The backup platen 90 is white at the scanning station to refleet any light passing through the document back through the document to the xerographic drum 22. The lamps LM P-l are housed in a housing 92 and shielded from the document by a transparent plate 94.

An optical image of the illuminated portion of the document at the exposure station 84 passes through the transparent plate 94, through an opening 96 in the lamp housing 92 to a combination mirror-lens assembly 98. The latent image passes through the lens 102 and is reflected by a mirror surface 104 back through the lens 102 to a mirror 106. The lens 102 focuses the image so that the image is properly focused on the drum surface 22 when it is reflected by the mirror 106 through the exposure slit 168 at exposure station 30 to the drum surface 22. The light path of the optical image is indicated generally by the dotted line 110.

The xerographic mechanism is shielded from the ambient light produced by the exposure system by means of a shield 112. The light shield 112 has an opening 114 positioned between the mirror 1% and the exposure slit 108 to allow passage of the light image to the drum surface. There is also an opening 116 in the light shield 112 which permits some of the ambient light to pass through the shield and be reflected by a reflective surface 113 to the drum to produce the discharge spot 62. The light of the discharge spot 62 removes any residual charge remaining on the drum surface after drum cleaning by rendering the photoconductive surface conductive and draining off any remaining charge.

The development system used to develop the latent electrostatic images on the side of the drum which is moving from bottom to top is a paddle wheel developer. A rotatable shaft 120 having a series of paddles 12-2 extending outward from the periphery thereof constitutes the paddle wheel 34. The paddle wheel 34 extends across the length of the drum 23 and is contained within the developer housing 124. This type of developer is also shown in FIGS. 2 and 6. The developer housing 124 extends from a point just past the exposure station 30 in the direction of movement of the drum to a point further along in the direction of rotation of the drum. The housing is shaped to contain a quantity of developer material 36 in contact with the paddle wheel 34. A developer 36 consists of carrier beads and toner particles intimately mixed so that a triboelectric charge causes the toner particles to adhere to the carrier beads. As the paddle wheel 34 rofates in the direction indicated by the arrow, the developer material is hurled or thrown by the paddles 122 upwards against the surface 22 of the drum 23.

Developer is prevented from passing between the drum surface and edge of the developer housing 124 by means of a drum seal 126. The seal 126 consists of a piece of nonconductive, nonabrasive material mounted on the edge of the housing 124 in contact with the surface of the xerographic drum. The seal 12d prevents carrier beads from escaping from the developer housing 124. At the time that the drum passes the edge of the housing, there is an electrostatic image on the surface which must be protected. Any seal used at this point must not only protect the electrostatic image but must also be free from any injurious effects to the photoconductive surface. Also, these characteristics of the seal must not change appreciably after extended use. For example, the seal cannot become abrasive or conductive after toner has had an opportunity to accumulate on the surface. A thin strip 127 of polyethylene terephthalate, sold under the trademark Mylar, of about .003 inch thickness is secured to a clamping member 12% mounted on the housing 124. The strip 127 extends into contact with the drum 22. Alternatively, the strip 127 may be constructed of polyurethane or fibrous web material of the type used in the web cleaner.

As developer encounters the surface of the drum, portions of the toner material are jarred loose from the carrier beads and are attracted by electrostatic charges on the drum surface. The electrostatic charges on the drum surface define the images to be reproduced, and thus the powder particles held on the drum surface by the electrostatic charge are in image configuration. The

carrier beads continue upward in their path of travel and eventually fall back into the lower portion of the developer housing 124 to be remixed with the main body of developer material. As toner particles are removed by the electrostatic charge on the drum surface, additional toner must be added to the main body of developer material. A toner dispenser 128 is mounted in the top of the developer housing 124 for the purpose of supplying additional toner material as needed. The toner dispenser 128 consists of a container portion 13% adapted to hold solid blocks of toner material 132, a pressure cap 134 and a scraping blade 136. The blocks of toner 132 are placed in the container with one edge against the blades 136. The cap 134 is also placed inside the container 130 against the blocks 132 and is held in place and forced forward by a series of leaf springs 138. The leaf springs 13 8- are mounted on brackets on the front cover 142 of the apparatus. As the cover 142 is closed, the leaf spring 140 is forced against the cap 134 urging the solid blocks of toner 132 against the blades 136.

Operation of the toner dispenser can best be seen in FIGS. 2 and 3. The blades 136 are similar to hack saw blades bent in a honeycomb configuration to insure complete coverage of the surface of the toner blocks and mounted in a pair of end blocks 144. The end block-s 144 are supported in the machine frame plate by means of a pair of sliding pins 14-6 at one end and a single pin number 148 at the other end. The pin "14 8 has a button or cap 150 mounted on one end so that it extends outside of the entire machine. A compression spring 152 is mounted on the shaft 148 between the cap 150 and a bearing plate 154. Pressure on the cap 152 forces the pin 148 and the blades 136 forward against the action of the spring 152. The movement of the blades 136 across the surface of the block 132 produces a scraping action which removes portions of the toner material from the block which then falls into the bottom of the developer housing 124. When the cap 150 is released, the spring 152 returns the springs 136 to their original position again producing a scraping action on the surface of the blocks 132 and removing toner therefrom.

A conveyor belt 43 is positioned to carry a sheet of paper from the copy paper opening 18 into contact with the xerographic drum 23 and then to the fuser 50 and ejection rollers 52. The belt 48 extends around a roller 156 just inside the copy paper opening 13 and a spring loaded tensioning roller 158 positioned just ahead of the heat fuser 5t). As the belt extends between the rollers 156 and 158, the bottom surface thereof is in contact with the top of the xerographic drum 23. The roller 158 is journaled in a bifurcated member 160 and is spring loaded by compression spring 162. The bifurcated member 160 is mounted on and pivoted about a shaft 16 so that rotation of the shaft 164- will elevate the roller 158 and the belt system 148 away from the drum 23. When a sheet of paper is inserted in the opening 18 on counter 166, it slides forward until it is gripped between a pressure roller 168 and the belt 48 at roller 156. The paper is carried forward in contact with the belt to a point just beyond an electrostatic discharge member or corotron 49. The belt 48 is held fiat at this point by a guide 170 mounted against the inside surface of the belt. The corotron 49 is energized when the paper hits switch LS1 and placed an electrostatic charge on the paper surface, in this case a positive charge, causing the paper to adhere to the belt. The electrostatic attraction between the paper and the belt causes the paper to be carried forward with the belt into surface contact with the xerographic drum. When the paper has been in contact with the xerographic drum and had a powder image transferred thereto, it is carried forward to the heat fuser 50 and is separated from the belt 48 by the sharpness of the angle that the belt takes around the roller 158. The sharpness of the angle and the natural rigidity of the paper are sufficient to overcome the electrostatic attraction between the belt and the paper, and thus the paper continues forward while the belt is directed around the roller 158.

The transfer corotron 40 mounted above the belt 43 at the top of the drum or transfer point 38 is tilted at a slight angle such that the electrostatic charges placed on the belt directly above the transfer point and not preceding the transfer point. When the electrostatic charge is placed on the belt slightly before the transfer point, the electrostatic image on the drum surface tends to produce an are between the drum and the belt; the powder image on the drum surface is disrupted or destroyed. Tilting the transfer COlGilOIl 40 in the manner shown in FlG. l eliminates the arcing prior to the transfer process.

In operation, the paddle wheel, indicated generally as 2%, is driven in a rotary manner during operation of the apparatus, that is, during movement of the Xerographic drum 22. Any suitable drive system capable of rotating the paddle wheels at desired speeds may be used. A schematic representation of a suitable drive system is shown in FIG. 2. The paddles 122 of the paddle wheels extend into the reservoir of developer material 36 so that rotation of the paddle wheel in the direction indicated by the arrows will lift developer materials from the reservoir and impel it towards the surface of the xerographic drum. The embodiments shown in FIGS. 5 and 6 impel the developer material against the drum surface in the direction that the surface of the drum is traveling. In the embodiment shown in FIG. 4, the developer material is impelled or hurled against the surface of the drum in both directions, that is, in the direction of travel of the drum surface and counter to the direction of travel of the drum surface. The relative speed between the movement of the carrier beads of the developer material and the surface of the xerographic drum may be controlled by varying the speed of rotation of the paddle wheels. Thus there is a high degree of flexibility in the speed at which the drum may be rotated. That is, a greater number of inches of drum surface per second or per minute may be developed by increasing the speed of the drum and proportionately increasing the speed of the paddle wheel without changing the relative speed of the developer material and the surface of the Xerographic drum.

The developer material is brought into contact with the surface of the Xerographic drum much in the same nanner as it is with conventional cascade development. The toner particles adhering to the carrier beads by triboelectric force are either jarred loose from the carrier beads upon impact with the suface of the Xerographic plate or are attracted from the carrier beads by the greater electrostatic charge on the surface of the xerographic plate. In any event, the electrostatic charge producing the latent electrostatic image on the drum surface is sufficient to attract the oppositely charged toner particles to the image areas and thereby produce a developed Xerographic powder image. The carrier beads fall away from the surface of the xerographic drum back into the main body of the developer material. Agitation of the main body of developer material by the paddle wheels causes a mixing action which stirs toner added from the toner dispenser 128 into the main mass of developer material and provides the triboelectric force for the carrier beads necessary to attract and hold the toner powder. The agitation of the developer material by the paddle wheel also produces loose toner particles in the atmosphere inside of the housing 124. This loose toner particle creates a powder cloud at the top of the developer housing. Since cascade development is particularly well adapted for the development of line copy and powder cloud is particularly suited for development of tone images, the apparatus shown herein is capable of developing line copy and tone images simultaneously.

In normal cascade development, carrier beads which cascade over the surface of the xerographic plate give up a quantity of the powder or toner adhering thereto at their first contact with the xerographic plate. The toner depleted carried beads continue to cascade across the surface of the xerographic plate and tend to attract by their triboelectric charge toner particles which are adhering to the surface of the xerographic plate in non-image areas. These toner particles adhering to the plate in non-image areas either are being held there mechanically or are being held by very small residual charges from the original electrostatic charge. The triboelectric charge on the carrier particle is sufficient to attract these particles away from the plate and thus remove what is termed background or undesirable toner deposits in non-image areas. The clean-up produced by the toner depleted carrier beads is referred to as scavenging. The scavenging effect present in cascade development is also present in the type of paddle wheel development shown in FIG. 4, i.e.. with twowheel paddle wheel developer. The paddle Wheel shown on the right operates counter to the direction of movement of the .lerographic drum and the images which are encountered by the carrier beads hurled against the surface of the plate by the paddle wheel have already been developed by the developer emanating from the left-hand paddle wheel and traveling in the direction of movement of the Xerographic plate. In the embodiment shown in FIG. 4, line copy is developed by the carrier beads hurled against the surface of the plate by the paddle wheels and tone copy is developed by the powder cloud formed in the space between the two paddle wheels where the two streams of developer material from the paddle wheels intersect or encounter each other and scavenging is produced after the drum has passed the powder cloud area, thus the device is particularly suited for all types of development and incorporates the best features of both cascade and powder cloud development.

While the invention has been described with reference to the structure disclosed herein, it is not confined to the details set forth; and this application is intended to cover such modifications or changes as may come within the purpose of the improvements or the scope of the following claims.

What is claimed is:

1. A developer apparatus for use in developing electrostatic images including a xerographic plate having a direction of movement,

a developer material consisting of a granular carrier material and a powder having a triboelectric relationship such that the powder is electrostatically held to the carrier material,

a housing capable of holding a quantity of developer material and having an opening adapted to receive the xerographic plate, said xcrographic plate having a charge pattern thereon, said charge pattern having a greater attraction for the powder than for the carrier,

impeller means in the housing with at least a portion thereof beneath the surface of the developer material and positioned to impel the developer material against the forces of gravity against the xerographic plate in the opening, the level of the developor material being below the top of the impeller means, said impeller means being spaced from the surface of the Xerographic plate, said impeller means impelling the developer material in the same direction of movement as xerographic plate,

and means to drive the impeller to agitate the developer material and to impel the developer material towards the opening in the housing.

2. A developer apparatus for use in developing electrostatic images including a xerographic plate having a direction of movement,

a developer housing containing a developer material consisting of a mixture of powder and granular carrier material having a triboelectric relationship such that the powder is electrostatically held to the carrier material,

the developer housing having one side open to receive a xerographic plate, said xerographic plate having a charge pattern thereon, said charge pattern having a greater attraction for the powder than for the carrier,

an impeller positioned in the housing and adapted to agitate the developer material and impel the developer material against the forces of gravity against the xerographic plate in the opening, the level of the developer material being below the top of the impeller means, said impeller means being spaced from the surface of the xerographic plate, said impeller means impelling the developer material in the same direction of movement as the Xerographic plate,

the opening in the developer housing being located so that the carrier material returns to the developer material after contacting the plate,

and means to drive the impeller at a speed sufiicient to impel the developer material toward the opening to thereby contact a xerographic plate in the opening with developer material.

3. A developer apparatus for use in developing electrostatic images including a xerographic plate having a direction of movement,

a developer housing containing developer material consisting of a mixture of developer powder and granular carrier material having a triboelectric relationship such that the powder is electrostatically held to the carrier material,

the developer housing having one side open to receive the xerographic plate, said xerographic plate having a charge pattern thereon, said charge pattern having a greater attraction for the powder than for the carrier,

at least one rotatable paddle wheel positioned within the housing partially submerged in developer powder 10 and granular carrier, the level of the developer material being below the top of the paddle wheel, said paddle wheel being spaced from the surface of Xerographic plate,

and means to rotate the paddle wheel at a speed sutficient to impel the developer material toward the opening to thereby contact the xerographic plate in the opening with developer material.

4. The apparatus of claim 2 wherein the impeller comprises a paddle wheel rotatably journaled in the sides of the developer housing and having the paddle extending down into the developer material.

5. The apparatus of claim 1 wherein the impeller means includes a pair of paddle wheels journaled in the developer housing and having the paddle wheels extending down into the developer material tothereby impel developer material against a Xerographic plate in the opening from two directions, said paddle wheels rotating in counter-rotating directions as to have the upper surfaces of the paddle wheels approaching each other.

References Cited UNITED STATES PATENTS 3,008,826 11/1961 Mott et al. 961 3,133,834 5/1964 Sowiak .1 18-637 2,221,776 1 1/ 1940 Carlson 118637 X 3,071,645 1/1963 McNaney 118637 X 3,081,737 3/1963 Frantz et al. 118-6 37 FOREIGN PATENTS 210,744 1/ 1960 Austria. 927,928 6/1963 Great Britain.

r CHARLES A. WILLMUTH, Primary Examiner. O P. FELDMAN, Assistant Examiner. 

